Find the inverse of the matrix, $\text B = \left[\begin{array}{rr}1 & 4 \\ 4 & 9\end{array}\right]$. Non-integers should be given either as decimals or as simplified fractions. $ B^{-1}=$
Answer: The Strategy To find the inverse of an invertible matrix, we can use Gaussian Elimination. To do this, we do the following. First, we append the matrix $\text B$ with the identity matrix $\text I$ to get [ B | I ] \left[\begin{array} ~\text B ~ |~\text I\end{array}\right]. Next, we use Gaussian Elimination to reduce $\text B$ to the identity matrix, $\text I$. Performing the same operations on $\text I$ will convert it to $\text B^{-1}$, so that our new matrix becomes [ I | B − 1 ] \left[\begin{array} ~\text I ~ |~\text B^{-1}\end{array}\right]. Appending $\text B$ with $\text I$ [ B | I ] = [ 1 4 4 9 1 0 0 1 ] \left[\begin{array} ~\text B ~ |~\text I\end{array}\right]=\left[\begin{array}{rr}1 & 4 & 1 & 0 \\ 4 & 9 & 0 & 1 \end{array}\right] Eliminating the leading term in the second row We want the first term of $R_2$ to equal $0$, so we subtract $4R_1$ from $R_2$. $\left[\begin{array}{rr}1 & 4 & 1 & 0 \\ {4} & {9} & {0} & {1} \end{array}\right]\xrightarrow{R_2-4R_1\rightarrow R_2}\left[\begin{array}{rr}1 & 4 & 1 & 0 \\ {0} & {-7} & {-4} & {1} \end{array}\right]$ Reducing the leading terms and back-solving Now, let's reduce the leading term of $R_2$ to equal $1$. $\left[\begin{array}{rr}1 & 4 & 1 & 0 \\ {0} & {-7} & {-4} & {1} \end{array}\right]\xrightarrow{-\dfrac{1}{7}R_2\rightarrow R_2}\left[\begin{array}{rr}1 & 4 & 1 & 0 \\ {0} & {1} & {\dfrac{4}{7}} & {-\dfrac{1}{7}} \end{array}\right]$ We are ready to back-solve to get [ I | B − 1 ] \left[\begin{array} ~\text I ~ |~\text B^{-1}\end{array}\right]. $\begin{aligned}\!\!\left[\begin{array}{rr}\!{1}\! & {4}\! & {1}\! & {0} \\ \!0\! & 1\! & \dfrac{4}{7}\! & -\dfrac{1}{7} \!\end{array}\right]\!\xrightarrow{\!\!R_1-4R_2\rightarrow R_1\!\!} \!\!&\left[\begin{array}{rr}\!{1}\!\! & {0}\!\! & {-\dfrac{9}{7}} \!\!& {\dfrac{4}{7}} \!\\ \!0\! & 1\! & \dfrac{4}{7}\! & -\dfrac{1}{7}\end{array}\right] \end{aligned}$ Therefore $\text B^{-1}=\left[\begin{array}{rr} -\dfrac{9}{7}\!\! & \dfrac{4}{7} \\ \dfrac{4}{7} \!\!& -\dfrac{1}{7} \end{array}\right]$. Summary $\text B^{-1}=\left[\begin{array}{rr} -\dfrac{9}{7}\!\! & \dfrac{4}{7} \\ \dfrac{4}{7} \!\!& -\dfrac{1}{7} \end{array}\right]$